The present invention claims priority to priority document no. 2001-051611 filed in Japan on Feb. 27, 2001, and incorporated by reference herein.
1. Field of the Invention
The present invention relates to a solderability testing apparatus and a solderability testing method.
2. Description of the Related Art
To achieve a desirable soldering, enough metallic bonding should be formed between a metal composing a base material (e.g. copper foil (or land portion) on printed circuit boards, and electrode portion of surface mounted devices) and solder (which is generally made of an alloy of tin and lead. It is thus necessary to ensure wetting of the surface of the base material with the solder (more specifically, tin). The wetting solder (tin) diffuses into the base material and forms therein an alloy layer formed with such base material through metallic bond, which is a final form of the soldering. So that testing the wetting balance between the base material and the solder can provide a quantitative evaluation of solderability.
Known apparatuses for evaluating wetting balances of solder, flux, solder alloy, solder paste (also referred to as cream solder) and so forth provided to lead portions of lead parts, lead and electrode portions of surface mounted parts, or land portions on printed circuit boards include an apparatus disclosed in Japanese Laid-Open Patent Publication No. H7-72064, and an apparatus specified based thereon by Standards of Electronic Industries Association of Japan (EIAJ) ET-7404, xe2x80x9cMethod for Testing Solderability of Surface Mounted Parts Using Solder Paste (Equilibrium Method)xe2x80x9d. The solderability testing apparatuses disclosed in these documents are suitable for solderability testing based on the equilibrium method.
As schematically shown in FIG. 1, the solderability testing apparatus specifically comprises a sample parts holding means 20, an external force detection means 10 for supporting such sample parts holding means 20, a solder paste container 30, and a heating means 40. The external force detection means 10 has a load cell (high-sensitivity load sensor). The solder paste container 30 contains a solder paste 31 which is internally added with a flux. The sample parts holding means 20 comprises a sample parts holding member 23 for holding a sample (or standard test piece) 50, an expansion sliding portion 21 for supporting such sample parts holding member 23, and an electromagnetic clutch 22 for locking such sliding portion 21. The sliding portion 21 is suspended at the upper end thereof from the external force detection means 10.
The solder paste container 30 is supported by a holder 32, and such holder 32 can ascend or descend, together with the solder paste container 30, with the aid of a stepping motor 33. The heating means 40 has a solder bath 41 which serves as a heat source. Solder 42 contained in the solder bath 41 is heated by a heater 43 to be brought into a molten state. The temperature of the solder 42 is monitored with a temperature sensor (e.g., thermocouple), not shown, and results of the measurement are fed back to control the heater 43. This allows the molten solder 42 in the solder bath 41 to be kept at a predetermined temperature. By dipping the solder paste container 30 in the solder bath 41 containing the molten solder 42 then successfully heats the solder paste 31 contained in such solder paste container 30 to thereby keep the molten state thereof at a predetermined temperature. The solder bath 41 can be ascend or descend with the aid of the stepping motor 44 provided thereunder.
FIG. 6A is a partial schematic view of the sample parts holding member 23 in a state of holding a sample 50 (e.g., surface mounted parts). The sample parts holding member 23 of the conventional solderability testing apparatus has been made of all sort of metals which can form structural member (except for those having a melting point of 500xc2x0 C. or lower, or those possibly act as a solder poison such as zinc and aluminum), which can be typified by steel and stainless steel material.
In the solderability testing, the sample 50 is held by the sample parts holding member 23, the stepping motor 33 is activated so as to raise the holder 32 together with the solder paste container 30 containing the solder paste 31, and the lower end of the sample 50 goes into the solder paste 31 and finally reaches the bottom plane of the solder paste container 30. Thereafter the solder paste container 30 pushes the sample 50 upward while being raised by the ascending holder 32. Thus the sample parts holding member 23 moves upward within the sliding portion 21 of the sample parts holding means 20 as much as the length of ascending path of the sample 50. After the ascending of the holder 32 together with the solder paste container 30 comes to the end, the sliding portion 21 is locked by means of the electromagnetic clutch 22. The lower end of the sample 50 is now in contact with the bottom plane (upper bottom) of the solder paste container 30. The stepping motor 33 is then activated to descend the holder 32 together with the solder paste container 30. Thus the sample 50 is held so that the lower end thereof is dipped in the solder paste 31 to a predetermined depth so as to keep a predetermined gap between such lower end and the upper bottom of the solder paste container 30.
Since the external force detection means 10 composing the load cell is applied with a load which is ascribable to the weights of the sample parts holding means 20 and the sample 50, so that such load is canceled as a tare so as to attain a load-zero status.
Then the stepping motor 44 is activated to raise the solder bath 41. This allows the solder paste 31 contained in the solder paste container 30 to be quickly heated to a temperature of the molten solder 42 and brought into a molten state. In the melting process of such solder paste 31, acting forces exerted on the sample 50, which are typified by buoyancy attributable to the solder paste 31 and surface tension of the molten solder paste 31, are detected by the load cell which composes the external force detection means 10, and then output as electric signals.
In the process of soldering, acting forces effecting between the molten solder paste 31 and the sample 50 are considered as two ways; that are acting force f1 ascribable to the surface tension of the solder paste 31; and buoyancy f2 from the molten solder paste 31, while ignoring the weight of adhered solder. The acting force f1 and buoyancy f2 can be expressed by the following equations (1) and (2), respectively, where force (tension) directed downward is defined as positive force.
f1=xcex3l cos "THgr")xe2x80x83xe2x80x83(1) 
f2=xe2x88x92xcfx81vgxe2x80x83xe2x80x83(2) 
, where, meanings of xcex3, "THgr", l, xcfx81, v and g are respectively as follows:
xcex3=boundary tension between molten solder paste and flux;
"THgr"=contact angle of molten solder paste with sample;
l=outer peripheral length of a sample measured at contact plane with molten solder paste;
xcfx81=density of molten solder paste;
v=volume of displaced molten solder paste; and
g=gravitational acceleration.
When heating of the solder paste 31 starts, the surface of the sample 50 starts to be wet with the flux preliminarily mixed into the solder paste 31, where the flux is responsible for removing oxide film or foreign matters from the surface of the sample 50 to thereby clean such surface of the sample 50.
Then the solder paste 31 starts to melt, buoyancy ascribable to such molten solder paste 31 starts to effect, and wetting with such solder paste 31 also starts when the temperature of the sample reaches a predetermined level. The force F exerted on the sample 50 herein is expressed as an equation below.
F=f1+f2=xcex3l cos "THgr"xe2x88x92xcfx81vg 
Time-course of such acting force (expressed by a wetting curve), time-course of the heating process, and wetting status of the sample 50 are shown in FIG. 5. Heating of the sample 50 immersed into the solder paste 31 starts at a point A in FIG. 5. Status of the sample 50 and solder paste 31 is expressed with a reference alphabet (a) in FIG. 5.
After complicated processes between points A and B, which include run of the solder paste 31, evaporation of the solvent and wetting of the flux (where the acting force ascribable to surface tension of the flux becomes maximum at the point B), the solder paste 31 starts to melt at such point B. Upon beginning of the melting of the solder paste 31, the buoyancy attributable thereto begins to effect, which is detected as a repulsive force against the sample 50 (a downward force in FIG. 5). Such repulsive force becomes maximum at a point C, and the entire solder paste 31 completes the melting. Also the contact angle between the molten solder paste 31 and the sample 50 reaches maximum as schematically indicated by reference alphabet (b) in FIG. 5 ( greater than 90xc2x0). By this point of time, a condition for wetting of the surface of the sample 50 with the molten solder paste 31 is already satisfied, so that the wetting starts. The acting force (referred to as tension hereinafter) ascribable to the surface tension of the molten solder paste 31 increases at the point C and thereafter, and suction force for the sample 50 (upward force in FIG. 5) is begun to be detected. The acting force F becomes zero at a point D, where the buoyancy and tension are kept in balance with each other.
The contact angle attained at this point of time is 90xc2x0 as specifically indicated by reference alphabet (c) in FIG. 5. The contact angle then decreases below 90xc2x0 at the point D and thereafter, where the molten solder paste 31 creeps up the non-dipped portion of the sample 50 to thereby form a meniscus as specifically indicated by reference alphabet (d) in FIG. 5.
The time period from the start of the heating (the point A) to the point D is referred to as wetting time tw (zero-cross time). The point D can be obtained approximately as an intersection of the zero line and wetting curve shown in FIG. 5. It is to be noted now that load difference between the zero line and the initial line indicating the zero-load status before the testing is started represents the weight of the solder paste bonding (remaining) on the sample 50 at the time point when the testing is completed. The shorter the wetting time tw is, the better the wetting balance between the solder paste 31 and sample 50 becomes, which indicates better solderability.
When the heating of the solder paste 31 begins, wetting of the surface of the sample 50 with the flux mixed into such solder paste 31 begins. More specifically, the flux creeps upward on the surface of the sample 50, instantaneously invades into the micro-gap between the sample 50 and sample holding portion of the sample parts holding member 23, and may even instantaneously reaches the top portion of the sample parts holding member 23. Such status is schematically illustrated in FIG. 6B, where the portion having the flux crept thereon is shown with hatching for easy understanding. This is attributable to the surface tension of the flux. Once such creeping-up of the flux such that reaching the top portion of the sample parts holding member occurs, the load cell detects surface tension and weight of the flux excessively applied thereto and also detects acting force caused by abrupt movement of the flux, which adversely affect detection result of the acting force (disturbance of the wetting curve) to a considerable degree. Or acting force ascribable to the surface tension of the flux can heavily affect the acting force detected by the load cell. The load cell which composes the external force detection means 10 is designed to detect extremely small force (in the order of several mN(Newton) to ten and several mN). So that such invasion and creeping-up of the flux into or onto the unnecessary portions will prevent the load cell composing the external force detection means 10 from precisely detecting the acting force exerted on the sample 50. Such wetting of the flux will even be more abrupt and rapid as compared with that of the solder paste, so that it becomes difficult to determine wetting time tw. A wetting curve for an exemplary case with such event is shown in FIG. 7.
The wetting tw can generally be estimated by personal-computer-assisted analysis of detected output of the external force detection means 10 (more specifically, load cell). However for the case shown in FIG. 7, the personal-computer-assisted analysis will determine a point B as a wetting time tw (zero-cross time), while an actual one should be determined as a point A, which erroneously shortens the observed wetting time twxe2x80x2 than the actual wetting time tw. Wetting curves obtained for the case with such phenomenon lacks reproducibility and consistency as typically shown in FIG. 8, which makes it extremely difficult to resolve such problem in the analysis of the wetting time on the software basis. FIG. 8 shows six wetting curves, and two out of six are expressed in an overlapped manner.
It is therefore an aspect of the present invention to provide a solderability testing apparatus and a solderability testing method, both of which can completely prevent the unnecessary flux wetting such that the flux invades upwardly into the gap between the sample and sample parts holding member, or even creeps up to the top portion of the sample parts holding member, and can ensure precise measurement of the wetting time particularly for the case that small-sized SMDs (Surface Mounted Devices) such as those of 0603 type or 1005 type are tested.
A solderability testing apparatus according to first and second aspects of the present invention, and a solderability testing method according to first to third aspects of the present invention are essentially based on the Standards of Electronic Industries Association of Japan (EIAJ) ET-7404, xe2x80x9cMethod for Testing Solderability of Surface Mounted Parts Using Solder Paste (Equilibrium Method)xe2x80x9d.
The solderability testing apparatus according to a first aspect of the present invention for attaining the foregoing aspect is such that comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; wherein such apparatus has a flux wetting preventive layer at least on the surface of a sample holding portion of the sample parts holding member.
A solderability testing method according to a first aspect of the present invention for attaining the foregoing object is such that using a solderability testing apparatus comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; such apparatus having a flux wetting preventive layer at least on the surface of a sample holding portion of the sample parts holding member; wherein such method comprises a step of heating and melting the solder paste using a heating means while keeping a part of a sample, which is held by a sample parts holding member, being dipped therein, and measuring time-dependent changes in the acting force effected between the molten solder paste and the sample using the external force detection means.
In the solderability testing apparatus or solderability testing method according to the first aspect of the present invention, the flux wetting preventive layer may be provided at least on the surface of a sample holding portion of the sample parts holding member, or may be provided on the most or entire surface of the sample parts holding member. More specifically, the flux wetting preventive layer may typically be formed on the surface of the sample holding portion of the sample parts holding member and the portion above thereof (portion onto which the flux can creep up).
The solderability testing apparatus according to a second aspect of the present invention for attaining the foregoing object is such that comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; wherein a sample holding portion of the sample parts holding member is made of a material having a poor wetting balance in respect of the flux.
A solderability testing method according to a second aspect of the present invention for attaining the foregoing object is such that using a solderability testing apparatus comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; such sample parts holding member having a sample holding portion which is made of a material having a poor wetting balance in respect of the flux, wherein such method comprises a step of heating and melting the solder paste using a heating means while keeping a part of a sample, which is held by a sample parts holding member, being dipped therein, and measuring time-dependent changes in the acting force effected between the molten solder paste and the sample using the external force detection means.
In the solderability testing apparatus or solderability testing method according to the second aspect of the present invention, at least the sample holding portion of the sample parts holding member may be made of a material having a poor wetting balance in respect of the flux, while it is also allowable that the entire portion of the sample parts holding member is made of a material having a poor wetting balance in respect of the flux. It is also preferable to form the sample holding portion of the sample parts holding member with a material having a poor heat conductivity.
In the solderability testing apparatus or solderability testing method according to the first aspect of the present invention, the material composing the flux wetting preventive layer preferably has a contact angle È to flux of larger than 90xc2x0, and such material is preferably selected from polymer material, cermet and ceramic. The polymer material herein preferably has excellent heat resistance, wear-proof property and water repellency, where preferable examples of which include various engineering plastics such as fluorocarbon resin or derivatives thereof (e.g., polytetrafluoroethylene) and polyoxymethylene (POM) resins. The cermet or ceramic herein preferably has excellent water repellency. The cermet refers to a composite material obtained by sintering ceramic and metal powder, or a composite material composed of ceramic, metal and so forth, and examples of which include those in which metal such as iron (Fe), nickel (Ni), cobalt (Co), chromium (Cr), molybdenum (Mo) or the like is combined with silicon (Si), boron (B), various carbides (TiC, ZrC, B4C, WC, SiC, etc.), oxides (Al2O3, ZrO2, ThO2, etc.) or nitrides (Wxe2x80x94N, Moxe2x80x94N, TaN, Bxe2x80x94N, etc.). The ceramic can be exemplified by so-called new ceramics such as alumina, mullite, magnesia, forsterite, zirconia, titania, yttria or the like; vitreous materials such as borosilicate glass, potassium borosilicate glass or the like; quartz glass and phosphosilicate glass.
Possible methods for forming the flux wetting preventive layer depend on materials composing such layer, where examples thereof include such that dipping the sample holding portion of the sample parts holding member (or entire portion thereof if necessary) into solution of a material composing such flux wetting preventive layer, which is followed by drying; such that coating solution of a material composing such flux wetting preventive layer on the portion the flux wetting preventive layer is to be formed, which is followed by drying; such that spraying solution of a material composing the flux wetting preventive layer to a portion the flux wetting preventive layer is to be formed, which is followed by sintering or flame coating if necessary; such that coating (depositing) the material; and such that forming film of a material composing the flux wetting preventive layer on the portion the flux wetting preventive layer is to be formed by physical vapor deposition (PVD) process such as sputtering or vapor deposition, or chemical vapor deposition (CVD) process. It is also allowable to employ direct processing or forming of plastic materials.
In the solderability testing apparatus or solderability testing method according to the second aspect of the present invention, the material composing at least sample holding portion (or the entire portion if necessary) of the sample parts holding member preferably has a contact angle E to flux of larger than 90xc2x0, and such material is preferably selected from various engineering plastics such as polymer materials having excellent heat resistance, processability, wear-proof property and strength (e.g., fluorocarbon resin or derivatives thereof typified by polytetrafluoroethylene, and polyoxymethylene resins), and from the foregoing cermet or ceramic.
In the solderability testing apparatus according to the first and second aspects of the present invention, and in the solderability testing method according to the first and second aspects of the present invention (all of which may simply be referred to as xe2x80x9cthe present inventionxe2x80x9d hereinafter), the sample parts holding means preferably comprises an expandable sliding portion for supporting the sample parts holding member and an electromagnetic clutch for locking such sliding portion. The sliding portion is preferably suspended at the upper end thereof from the external force detection means; or the sample parts holding member, which is made of a flexible (springy) material, preferably has on the outer periphery thereof a sleeve covering thereof, and is suspended at the upper end of such sleeve or at the sample parts holding member per se from the external force detection means, although the present invention is by no means limited to these constitutions.
In the present invention, the external force detection means can be composed, for example, of a high-sensitivity load sensor such as load cell, or an electronic balance. The solder paste container can specifically be composed of testing jig plate I or II specified in EIAJ ET-7404. The heating means preferably has a heating bath containing liquid metal (e.g., solder bath containing solder, heating bath containing fusible alloy such as Wood""s metal), which can ensure a larger contact surface area than a heating plate can.
In the solderability testing method according to the first and second aspects of the present invention (generally referred to as xe2x80x9cthe solderability testing method of the present inventionxe2x80x9d hereinafter), the acting force effected between the molten solder paste and the sample is specifically understood as a synthetic force of buoyancy exerted on the sample from the solder in the molten solder paste and the surface tension (tension).
The present invention is applicable to evaluation of solderability of the samples, or solder wetting balance listed below:
(1) lead portion of lead parts;
(2) electrode portion (terminal portion) of surface mounted parts;
(3) land portion provided on printed circuit boards;
(4) flux;
(5) solder alloy (including lead-free solder such as Snxe2x80x94Cu and Snxe2x80x94Cuxe2x80x94Ag solders);
(6) solder paste (also referred to as cream solder);
(7) raw materials before being processed into part terminal leads or lead frames, such as wire material (Cu/CP wire), hoop material (to be processed into lead frame, terminal and so forth, and exemplified by 426 alloy, 42 alloy, etc.), steel sheet (tin-plated or zinc-plated steel plate such as tin plate or galvanized steel sheet);
(8) materials obtained by subjecting the materials described above in (7) to various plating processes (evaluation of solder wetting balance of plating chemicals);
(9) evaluation of solderability of films formed using surface treatment apparatuses such as PVD and CVD apparatuses on the materials described above in (7);
(10) performance test of solderability in apparatuses and methods of surface treatment such as PVD and CVD; and
(11) surface treatment materials (BTA, imidazole) other than flux in relation to the solderability.
For the evaluation of (1), (2) or (3), lead part, surface mounted part, printed wiring board, or land portion on such printed wiring board is assumed as the sample. In such cases, it is preferable to use for example a standard solder paste specified by EIAJ ET-7404, or a standard (reference) paste preliminarily defined by the user. When items (1) to (3), and (7) to (10) are to be evaluated, the evaluation preferably follows the solderability testing method according to the first or second aspect of the present invention. Conditions for such testing are preferably in compliance with the recommended testing conditions specified by EIAJ ET-7404, while not being limited thereto.
On the other hand, for the evaluation of item (4), it is preferable to use a copper oxide wire (0.6 mm in diameter) or a reference material defined by the user (e.g., electrodes subjected to lead-free plating with Snxe2x80x94Cu, Sn, Snxe2x80x94Ag or the like) as a standard sample, and to prepare the solder paste using a component solder powder specified by EIAJ ET-7404. For the evaluation of items (5), (6) and (11), it is preferable to use a copper oxide wire (0.6 mm in diameter) as a standard sample. When items (4), (5), (6) and (11) are to be evaluated, the evaluation preferably follows the solderability testing method according to the first or second aspect of the present invention.
In the solderability testing methods according to the first and second aspects of the present invention, temperature elevation profile attained in the process in which the solder paste is heated using a heating means to be brought into a molten state (elevation profile of sample temperature) preferably follows the rapid heating profile specified by EIAJ ET-7404, while being not limited thereto.
In the present invention, at least the surface of the sample holding portion of the sample parts holding member has formed thereon the flux wetting preventive layer, or at least the sample holding portion of the sample parts holding member is made of a material having a poor wetting balance, which is advantageous for surely preventing the flux from invading into the gap between the sample and sample holding portion of the sample parts holding member, and eventually from creeping up to the sample parts holding member. This ensures precise measurement of the wetting time.